1. Field of the Invention
The present invention relates to semiconductor lasers and, more particularly, to a surface emitting distributed feedback semiconductor laser with a chirped grating surface that provides high power efficiency and produces a nearly single-lobed longitudinal mode far-field beam profile.
2. Description of the Prior Art
Light amplification by the stimulated emission of radiation (laser) produces unidirectional, monochromatic, and most importantly coherent visible light. The stimulated emission of radiation is a process in which the energy state of an atom can change in a quantum transition with the emission of a photon. During this process, a photon approaches an atom, initially in an excited energy state, and induces this atom to make a transition to a lower energy state. As the atom's energy state is lowered, the atom emits a photon. This emitted photon, which is separate from the photon that induced the energy transition, possesses an energy that is equal to the difference between the excited and the lower energy states of the atom. Moreover, this emitted photon and the inducing photon both leave the atom in the same direction the inducing photon had as it approached the atom. These photons are also exactly in relative phase with one another; that is, they are coherent. This coherence is dictated by energy conservation in that if the two photons were out of phase by any amount they would interfere destructively, thereby violating energy conservation. Therefore, stimulated emission of radiation is a process that induces coherent photon multiplication or light amplification, thus a laser.
Laser technology has evolved by applying the above stated principle to several different types of active media. The most recent development in this field, coupled with the advancements in semiconductor fabrication technology, is the semiconductor laser. Unlike an atomic laser, however, stimulated emission in a semiconductor laser occurs when there is an excited state of a solid state material, thus it involves more than one atom.
A surface emitting distributed feedback semiconductor laser is a device that produces unidirectional, monochromatic, coherent visible light through stimulated emission in semiconductor materials. This device has a positively doped side and a negatively doped side that are joined at a junction, and a grating surface that is fabricated onto an outer surface of the positively doped side. The grating surface, fabricated with a strong conductive material, provides a means by which coherent photon energy fields may be diffracted. A second order grating design permits deflections of coherent photon radiation to be directed normal to an output window etched into the negatively doped side of the junction through first order diffraction, and directed parallel to the grating surface through second order diffraction. The first order diffraction produces a beam of unidirectional, monochromatic, coherent visible light at the output window, whereas the second order diffraction provides a feedback of photon radiation to an active region that is adjacent and parallel to the uniform grating surface.
A theoretical longitudinal mode near-field intensity profile produced at the output window of a surface emitting distributed feedback semiconductor laser device is antisymmetric with a zero intensity null at the output window center. A corresponding theoretical longitudinal mode far-field intensity profile is double-lobed and symmetric about the output window center. These theoretical intensity profiles have been practically demonstrated in actual device measurements, although in these measurements it is found that spontaneous emission partially fills the near-field center null. Much has been written on the subject of semiconductor lasers in recent years and some good descriptive background articles on these devices are Surface Emitting Distributed Feedback Semiconductor Laser, Applied Physics Letters, Volume 51, Number 7, pp. 472-474, August 1987, and Analysis of Grating Surface Emitting Lasers, IEEE Journal of Quantum Electronics, Volume 26, Number 3, pp. 456-466, March 1990.